Eyeglasses are frequently damaged by inadvertent pressure being applied to the temple member relative to the lens frame as, for example, when the temple members are opened and when weight is applied to the eyeglasses. Another reason for failure of an eyeglass hinge is the plastic deformation of the temple member, lens frame, and hinge from the fatigue of the cyclic stress experienced in the putting on and removing of the eyeglasses over a period of time.
Another mode of eyeglass hinge failure is the loss or loosening of small screws, serving as hinge pins, which tend to come loose and are easily lost. These small parts often require specially designed tools to effect repairs. When the screws become loose, or the parts become plastically deformed, the temple member can open beyond its designed position thereby reducing or losing its function of applying lateral inward pressure to the wearer, and therefore losing its ability to hold the eyeglasses securely to the wearer's head.